1. Field of the Invention
This invention relates to a heat pump type hot-water supply system.
2. Description of the Related Art
JP-A-10-89816 discloses a heat pump type hot-water supply system. This system changes a flow direction of refrigerant circulating in a refrigerant circuit between a hot-water supply operation and a defrosting operation. Accordingly, a water heat exchanger that works as a condenser at the hot-water supply operation is used as an evaporator at the defrosting operation, while an exterior heat exchanger that works as an evaporator at the hot-water supply operation is used as a condenser at the defrosting operation. Heat of hot water heated at the hot-water supply operation is absorbed by the water heat exchanger, and is radiated in the frosted exterior heat exchanger to perform defrosting of the exterior heat exchanger.
In an air conditioning system disclosed JP-B2-7-99297, a bypass pipe is branched from a discharge side pipe of a compressor, and is connected to an inlet side pipe of an exterior heat exchanger. Accordingly, part of high-temperature gas discharged from the compressor is conducted into the bypass pipe to perform defrosting of the exterior heat exchanger.
In the conventional systems, however, plural functional parts must be provided for selecting the hot-water supply operation or the defrosting operation. Specifically, the system using the exterior heat exchanger as the condenser at the defrosting operation requires a four-way valve and several switching valves for changing the refrigerant flow direction. The system in which high-temperature gas discharged from the compressor is conducted directly into the exterior heat exchanger requires the bypass pipe, several switching valves, and the like. Therefore, system cost is increased in theses systems, and the cycle constitutions are complicated, thereby lowering reliability of the systems.
Especially in a case where the heat pump cycle uses CO2 as refrigerant and pressurizes it up to a critical pressure or more, the four-way valve and the switching valves for opening or closing the refrigerant passage are required to have high accuracy sealing properties resistant to high pressure. Such valves are very expensive, and further increases the system cost.
The present invention has been made in view of the above problems. An object of the present invention is to provide a heat pump type hot-water supply system capable of performing defrosting operation at low cost and high reliability of the system.
In a heat pump type hot-water supply system according to the present invention, at a defrosting operation, an opening degree of a pressure-reducing device is controlled to be larger than that at an ordinary operation at which liquid stored in a reservoir is circulated and is heated in a first heat exchanger for supplying hot gas. Further, a pump for circulating liquid is stopped. Accordingly, high-temperature refrigerant (hot gas) discharged from a compressor radiates a decreased amount of thermal energy in the first heat exchanger. A decrease in pressure of hot gas in the pressure-reducing device is also suppressed. As a result, hot gas discharged from the compressor can reach a second heat exchanger without lowering its temperature largely and perform defrosting of the second heat exchanger. These effects can be provided with a simple cycle structure at low cost.
At the defrosting operation, liquid heated and stored in the reservoir may be supplied into the first heat exchanger in the state where the opening degree of the pressure-reducing device is larger than that at the ordinary operation. In this case, heated liquid raises temperature of the body of the first heat exchanger. Accordingly, the decrease in temperature of hot gas in the first heat exchanger can be suppressed, thereby suppressing thermal loss of hot gas in the first heat exchanger. The defrosting time is therefore shortened.
Heated liquid may be supplied into the first heat exchanger continuously during the defrosting operation. In this case, first, the thermal energy of heated liquid is consumed to raise the temperature of the first heat exchanger body. After that, as soon as the temperature of the first heat exchanger body is increased not to cause thermal loss of hot gas, the liquid is used to heat hot gas. Accordingly, defrosting of the second heat exchanger can be performed for a further shortened time.